I wanted to be able to easily type a combination of English and Spanish and for my spelling to be checked for both simultaneously.

So, I installed two language files. I then combined my primary English dictionary with the Spanish (Spain) dictionary available from the Firefox add-on site. I combined the files by copying and pasting the contents of my English .aff and .dic files into the the Spanish .aff and .dic files.

This sort of works. I say sort of because when I use the Spanish dictionary, most English words are correctly identified as being spelled correctly. However, plural words and words ending in ed are identified as spelled incorrectly. Here is a screenshot of the phenomenon I’m describing.

In Windows 7, the dictionary files are installed here: C:\users\[User Name]\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxxx.default\